Tandem mass spectrometry or MS/MS is a method which includes dissociation of selected precursor ions followed by mass analysis of the resultant product ions. MS/MS can be used to identify a precursor ion and determine its structure. It is commonly used for structural analysis of a wide variety of compounds, including peptides, proteins and oligopeptides.
In practice, tandem mass spectrometry apparatus includes means for selecting precursor ions, means for dissociating the selected precursor ions and means for further mass analysis of the resultant product ions. Several different design approaches may be adopted for this purpose. Some designs, such as those based on triple quadrupole (TQ), magnetic sector, or time of flight (ToF), require separate instrumentation dedicated to carrying out the respective function at each successive stage of the MS/MS process. However, the most attractive design for tandem mass spectrometry is based on a quadrupole radio frequency ion trap (QIT). A QIT can be used to select precursor ions and confine the selected ions within a defined spatial volume, enabling one or more stages of dissociation and product ion analysis to be carried out. Methods such as collisionally induced dissociation (CID) and photo dissociation (PD) have been used to dissociate precursor ions in an ion trap device. Surface induced dissociation (SID) is another dissociation technique whereby dissociation is brought about by collision of the ions with a surface. This technique has a relatively high dissociation efficiency, typically up to 50%.
Conventional methods for exciting ion motion in an ion trap device involve exciting the ion cloud in the axial direction only. Such excitation is commonly used as a method for heating the ion cloud to encourage CID. The maximum energy which can be released by the collision of an ion with a neutral molecule is a function of the masses of the two particles. When averaged over the Maxwellian distribution of buffer gas velocity, this energy <Ecoll> can be expressed as:                     <                  E          coll                >=                                            3              ⁢              kT                        2                    +                                    M              b                                      M              i                                      <                  K          i                >                            (        1        )            where T is the temperature of the buffer gas, Mb and Mi are the masses of the buffer gas molecule and of the ion respectively and <Ki> is the average kinetic energy of the ion. The kinetic energy of ions in the ion trap device is limited. It follows from equation (1), that the CID process is ineffective for heavy ions. By contrast, the total kinetic energy of an ion may be transformed into internal degrees of freedom when the ion collides with an electrode surface. Thus, the SID process, which exploits such collisions has the advantage that its effectiveness is not constrained by the mass of the precursor ion.
Excitation of the ion cloud in the axial direction is unsuitable when SID is being used because the end cap electrodes at which collisions would occur have entrance and exit holes reducing the effectiveness of the process. It is preferable to induce collisions at the ring electrode.